1. Field of the Invention
The present invention generally relates to a forward switching mode power converter. More particularly, the present invention relates to a PFC-PWM controller having a power saving means. The present invention teaches a forward power converter with power factor correction in the front end.
2. Description of the Related Art
Switching-mode power converters have been widely used in many electronic appliances over the last few decades. Switching-mode power converters have several advantages over linear power converters, including higher power conversion efficiency, lighter weight, and smaller size.
However, traditional switching-mode power supplies have some drawbacks. A typical switching mode power converter conducts a non-sinusoidal line current in short pulses that are in phase with the line voltage. This is undesirable for a switching-mode power supply, because it reduces the power factor. Ideally, a switching mode power converter should have a power factor close to 1, but non-sinusoidal line current conduction reduces this to approximately 0.6. For applications that consume 70 watts or more, this could be a serious source of power loss.
Another drawback of switching mode power converters is that line harmonics are produced whenever the line current is not sinusoidal. The harmonic currents do not contribute to the load power, but they do cause excess heat generation in the power contribution system.
To avoid unnecessary power losses and heat dissipation, some prior-art switching-mode power converters include power-factor correction circuitry. Power-factor correction is applied to the line current to create a sinusoidal input current waveform that is in phase with the line voltage.
One drawback of traditional power factor correction schemes is that they do not reduce power consumption sufficiently to comply with recent environmental regulations. In recent years, many countries have adopted strict regulations regarding power consumption. Electronic devices that consume 70 watts or more are generally required to minimize power consumption during standby, or idle-mode. However, traditional switching-mode power supplies with power factor correction still operate at a specific PWM switching frequency during standby. Since the power consumption of a switching-mode power converter is directly proportional to the switching frequency of the PWM signal and the PFC signal, prior-art switching-mode power supplies fail to minimize power consumption during standby.
Recently enacted environmental regulations regarding standby mode power consumption have created a need in many countries for more efficient power supplies. A switching-mode power supply having power factor correction and a switching frequency reducing means under light-load and zero-load conditions is needed.